The present invention is directed to a dry (oil free) piston type vacuum pump and more specifically to a vacuum pump having two piston and cylinder assemblies operatively connected to a common drive shaft with each piston having a pair of gapless full contact lip seals engaging the cylinder and an outlet valve plate which covers the entire piston diameter and which makes full face contact with the piston to minimize dead volume between the piston and the valve plate.
At present, pre-pumping to a rough vacuum is usually carried out by an oil-seal rotary pump which is both lubricated and sealed with hydrocarbon or fluorocarbon oil. Some of the oil molecules are degraded and fragmented into smaller molecules during the operation of the rotary pump and the small hydrocarbon and fluorocarbon molecules exhibit a high vapor pressure relative to that of the oil before the latter was used in the pump. It is difficult to prevent the small molecules from passing back from the pump and entering the vacuum vessel where they contaminate all the surfaces of the vessel and the contents by coating them within an adherent oily film. Such a film is completely unacceptable in many high technology areas. Further problems are associated with the maintenance of the correct oil level and the disposal of used oil. Such maintenance is time consuming and costly.
In order to provide an oil free pump an attempt was made to utilize split polytetrafluoroethylene (PTFE) sealing rings backed by split, spring-steel bands or other expansion means. However, it was impossible to achieve a high vacuum with such pumps due to the inevitable leakage due to the split. In order to overcome this problem, a clearance seal was developed wherein a sleeve of low friction material was disposed on the cylindrical surface of a piston head such that over the temperature range encountered during normal operation of the pump, a mean gap was sustained about the sleeve between the sleeve and the cylinder, which gap is of a maximum size at which leakage of gas past the sleeve is at a level acceptable for a vacuum to be sustained by the pump. Such a clearance type seal is disclosed in the U.S. Pat. No. 4,790,726 to Balkau et al.
Another problem encountered with vacuum pumps resided in the fact that at extremely low pressures the pressure of the gas compressed by the cylinder might be insufficient to open the exhaust valve at the top of the cylinder. Accordingly, valves were designed to open upon contact with the piston at the top dead center position of the piston to facilitate the exhausting of the compressed gas. Such a valve is disclosed in the U.S. Pat. No. 4,790,726 to Balkau et al. as well as the U.S. Pat. No. 4,854,825 to Bez et al. both of these patents disclose a portion of a valve member protruding into the cylinder for contact by the piston to open the valve. However, a considerable volume of dead space still remained between the piston and the cylinder head causing the ultimate pressure to remain high.